Liquid absorbent structures are well known in the art such as for the application in absorbent articles such as baby or adult incontinence diapers, feminine hygiene articles, but also for absorbing liquids in food packages or for other liquid spills.
It is also well known to use superabsorbent polymers (SAP). Whilst such materials provide improved liquid storage capacity as compared to the fibrous materials typically used before, they provided challenges to the designer of absorbent structures.
For example, such SAP are typically provided as particles. Thus, unless certain measures are taken, there is a risk of such particles dislocating, either during manufacturing, storage, or use, thereby at least altering the performance characteristics of the structure if not spilling out of the structures undesirably.
Further, even the most advanced versions of SAP are still prone to the “gel blocking” phenomenon, i.e. when used at high concentrations or as pure particle accumulation, some of the SAP can swell up whilst hindering liquid to reach other SAP particles.
There has been a plethora of approaches to overcome these and other challenges.
Starting from fibrous, mostly cellulosic, structures with relatively small amounts of SAP distributed therein, the properties of the SAP have ongoingly been improved so as to allow concentrations of about 60% by weight without substantial gel blocking occurring. Also, at such concentrations the fibrous matrix dominates the structures and holds the particles sufficiently in place, see e.g., EP0339461 (K-C; Kellenberger) or EP0752892A1 (P&G; Goldman).
Further it has been recognized and described in EP1982678 (P&G) that absorbent structures should not only immobilize the particles in the dry state, but that also the liquid loaded structure should immobilize the swollen gel.
In order to increase the relative amount of SAP in an absorbent structure, the cellulosic fibres were at least partly removed and replaced by synthetic binder.
An early approach is described in EP0297411A1 (Peaudouce, Koczab) by placing SAP particles in a matrix of synthetic fibres and thermobonding the structure along a bonding pattern with continuous bond lines forming a hexagonal pattern. The thermobonding has been described to negatively impact the absorbency properties of the structure.
EP1199059B1 (U-C; Onishi) describes the mixing of fibres exhibiting a lower and a higher melting point such that upon application of energy a three-dimensional fibre matrix is created.
In JP2008/125602 a structure is described wherein two types of fibres with particles intermixed are permanently bonded.
Further approaches position SAP particle accumulations in “pockets” sandwiched between webs. The particles can be immobilized such as by circumscribing the pockets with bonded regions, either by adhesive means or by thermobonding, see e.g.,
In EP1621165A1 (P&G; Blessing) structures are described that rely on adhesive bonds and aim by a rather complex process to keep the bonding regions between premetered SAP particle accumulations free of SAP particles.
WO2012/048878 (Romanova; van der Maele) shows premetering of SAP particle accumulations and thermobonding of the webs. Even though particular and complex measures, such as specific air blower arrangement, are taken to keep the boding regions free from particles, the bonding is sensitive to such contamination.
The description of the copending application GB12166708 (filed on Sep. 19, 2012; unpublished; C4S; Schmitz), provides an improved sandwich structure with particulate SAP pockets positioned between two ultrasonically bonded webs. The particular method ensures that the bond points are free of particles and thus the bond points provide strong bonds between the sandwich webs.
As SAP material swells upon imbibing liquids, it expands in volume, which has to be reflected in the structure.
Thus in U.S. Pat. No. 5,411,497 (K-C; Tanzer) structures are described with water-sensitive attachment, such that upon the first wetting of the structure such attachments de-bond and the SAP can swell more freely.
Notwithstanding the above approaches, there is still a need for providing absorbent structures which provide good dry and wet immobilization whilst not unduly deteriorating the absorbency properties of the structure compared to the ingoing materials.
Even further there is a need for providing a simple method for manufacturing such structures, without a need for dosed deposition of material pockets as described in the above mentioned publications.